There is class of devices that has been used for separating of whole blood into one or more of its constituents, such its cellular constituents (red blood cells and platelets) and its non-cellular constituent (plasma), based on the use of a membrane. More specifically, this type of device employs relatively rotating surfaces, at least one or which carries a porous membrane. Typically the device employs an outer stationary housing and an internal spinning rotor covered by a porous membrane.
One such well-known plasmapheresis device is the Autopheresis-C® separator available from Fenwal, Inc., a Fresenius Kabi company, of Lake Zurich, Ill. A detailed description of a spinning membrane separator may be found in U.S. Pat. No. 5,194,145 to Schoendorfer, which is incorporated by reference herein. This patent describes a membrane-covered spinner having an interior collection system disposed within a stationary shell. Blood is fed into an annular space or gap between the spinner and the shell. The blood moves along the longitudinal axis of the shell toward an exit region, with plasma passing through the membrane and out of the shell into a collection bag. The remaining blood components, primarily red blood cells, platelets and white cells, move to the exit region between the spinner and the shell and then are typically returned to the donor.
Spinning membrane separators have been found to provide improved filtration rates, due primarily to the unique flow patterns (“Taylor vortices”) induced in the gap between the spinning membrane and the shell. The Taylor vortices create shear forces in the gap that help to keep the cells, proteins or various biomolecules present in a biological fluid from depositing on and fouling or clogging the membrane.
When a membrane becomes fouled, the effective surface area of the membrane available for filtration is decreased. As membrane surface area decreases the separation efficiency of the device is reduced, and a higher transmembrane pressure (TMP) is required for filtration. It is often desired to prevent, or at least limit, fouling of a membrane. Some degree of fouling is acceptable, as it is representative of an aggressive filtration procedure. However, excessive fouling is indicative of over-aggressive filtration that will lead to pressure and efficiency related issues.
In order to control fouling, the controllers for the separation systems have been programmed with algorithms that monitor the fouling, which is measured as a change in the TMP over time. Monitoring the fouling rate (mmHg/min) provides the system with the information required to make a decision regarding the filtration procedure parameters that aims to reduce fouling in order to maintain fouling rates within predetermined limits.
U.S. Pat. No. 8,840,790, incorporated herein by reference and having the same assignee as the present application, discloses a method for controlling fouling which involves changing the outlet cellular concentration, and in turn the flow rate through the membrane, to control the risk of fouling. For a constant rotation rate and inlet flow rate to the spinner, controlling fouling by changing the outlet concentration directly impacts separation efficiency, with lower outlet concentrations leading to lower efficiencies (efficiency being characterized by the volume of supernatant filtered through the membrane from the cell suspension compared to the supernatant volume entering the spinner). Therefore, current practice, as exemplified by U.S. Pat. No. 8,840,790, can be considered a “variable efficiency” method for controlling fouling.
When the efficiency changes throughout a procedure, via changing the outlet concentration, the overall or averaged concentration of cells flowing out of the spinner can be different for each procedure. During plasmapheresis, in which plasma is separated from red blood cells (RBCs), this is not an issue, as the RBCs that exit the spinner are returned to the donor. Thus, it is not a problem if the concentration of RBCs, (or the hematocrit of the fluid returned to the donor), (and thus the efficiency of the separation) varies slightly throughout the procedure, as it will only affect procedure time. However, in applications that use the cells exiting the spinner as a product, it may be important to maintain a constant concentration (efficiency) throughout a procedure to ensure the products from procedure to procedure contain constant concentrations and volumes. Thus, there is a need for a “constant efficiency” method for controlling fouling of membrane separators during the separation of biological suspensions. The subject matter disclosed herein provides such a “constant efficiency” method.